Kerr Optical Frequency Comb Research Articles

Experimental Observation of Raman Assisted and Kerr Optical Frequency Comb in a 4H-Silicon-Carbide on Insulator Microresonator

Kerr nonlinear microcavities have garnered significant interest owing to their rich dynamics of nonlinear optical phenomena and compatibility with on-chip photonic integration. Recently, silicon carbide has emerged as a compelling platform due to its unique optical properties. In this study, we demonstrate Raman-assisted and Kerr optical frequency generation in a 4H-silicon carbide-on-insulator microresonator. By pumping the transverse electric (TE00) mode within the device, we observe a stimulated Raman scattering (SRS) Stokes with the Raman shift at approximately 775 cm-1, achieved with an on-chip power of 350 mW. Furthermore, by red-tuning the TE00 pump wavelength, we have achieved the coexistence of Raman and Kerr frequency combs. Using another device on the same chip with light variation of the taper we can observe the Raman and Kerr frequency combs within a spectral bandwidth ranging from ∼ 1440 to 1960 nm. The inclusion of the Raman-assisted comb extends the comb’s coverage into longer wavelength regimes, making it highly desirable for spectroscopy applications.

A review of gallium phosphide nanophotonics towards omnipotent nonlinear devices

Abstract Gallium phosphide (GaP) has been increasingly prioritized, fueled by the enormous demands in visible light applications such as biomedical and quantum technologies. GaP has garnered tremendous attention in nanophotonics thanks to its high refractive index, indirect bandgap width of 2.26 eV, lattice perfectly matched with silicon, and omnipotent and competitive nonlinear optical properties. Herein, we review the progress and application of GaP in nanoscale devices over the past two decades. The material properties of bulk GaP are first listed, followed by a summary of the methodologies for fabricating nanoscale devices and related integration techniques. Then, we digest the operational mechanisms across different GaP-based devices on their optical linear responses. Following this, we categorize the GaP nonlinear optical effects into multiple aspects including second-harmonic generation, four-wave mixing, Kerr optical frequency combs, etc. Ultimately, we present a perspective on GaP nanophotonics in the context of coexisting and competing modes of various nonlinear effects. We believe that a comprehensive overview of unique GaP will propel these nanophotonic devices toward a mature state, underpinning foundational understanding and leveraging practical innovations.

A perspective on nonlinear, microwave, and quantum photonics with Kerr microcombs

Microresonator Kerr optical frequency combs currently constitute a well-established research area in integrated, nonlinear, and quantum photonics. These systems have found a plethora of technological applications, while serving as an excellent platform to investigate fundamental scientific topics such as light–matter interactions, pattern formation in driven-dissipative systems, or entangled twin-photon generation. We here provide a brief overview of the topic, highlight some of the most recent advances, and discuss a few of the main challenges ahead in this field.

Multi-mode microcavity frequency engineering through a shifted grating in a photonic crystal ring.

Frequency engineering of whispering-gallery resonances is essential in microcavity nonlinear optics. The key is to control the frequencies of the cavity modes involved in the underlying nonlinear optical process to satisfy its energy conservation criterion. Compared to the conventional method that tailors dispersion by cross-sectional geometry, thereby impacting all cavity mode frequencies, grating-assisted microring cavities, often termed as photonic crystal microrings, provide more enabling capabilities through mode-selective frequency control. For example, a simple single period grating added to a microring has been used for single frequency engineering in Kerr optical parametric oscillation (OPO) and frequency combs. Recently, this approach has been extended to multi-frequency engineering by using multi-period grating functions, but at the cost of increasingly complex grating profiles that require challenging fabrication. Here, we demonstrate a simple approach, which we term as shifted grating multiple mode splitting (SGMMS), where spatial displacement of a single period grating imprinted on the inner boundary of the microring creates a rotational asymmetry that frequency splits multiple adjacent cavity modes. This approach is easy to implement and presents no additional fabrication challenges compared to an unshifted grating, and yet is very powerful in providing multi-frequency engineering functionality for nonlinear optics. We showcase an example where SGMMS enables OPO across a wide range of pump wavelengths in a normal-dispersion device that otherwise would not support OPO.

A stochastic approach to phase noise analysis for microwaves generated with Kerr optical frequency combs

Kerr optical frequency combs are expected to play a major role in photonic technology, with applications related to spectroscopy, sensing, aerospace, and communication engineering. Most of these applications are related to the metrological performance of Kerr combs, which is ultimately limited by their noise-driven fluctuations. For this reason, it is of high importance to understand the influence of random noise on the comb dynamics. In this communication, we theoretically investigate a model where Gaussian white noise is added to the coupled-mode equations governing the comb dynamics. This stochastic model allows us to characterize the noise-induced broadening of the spectral lines. Moreover, this study permits to determine the phase noise spectra of the microwaves generated via comb photodetection. In this latter case, our analysis indicates that the low-frequency part of the spectra is dominated by pattern drift while the high-frequency part is dominated by pattern deformation. The theoretical results are found to be in excellent agreement with numerical simulations.

Experimental demonstration of Kerr optical frequency comb generation in a tellurite microsphere.

We experimentally demonstrate optical frequency comb generation in a tellurite microsphere, for the first time to the best of our knowledge, for tellurite glass microresonators. The TeO2-WO3-La2O3-Bi2O3 (TWLB) glass microsphere has a maximum Q-factor of 3.7 × 107, which is the highest ever reported for tellurite microresonators. We obtain a frequency comb containing seven spectral lines in the normal dispersion range when pumping the microsphere with a diameter of 61 µm at a wavelength of 1.54 µm.

Flat-Top Soliton Frequency Comb Generation Through Intra-Cavity Dispersion Engineering in a Brillouin Laser Cavity

Dissipative Kerr cavity soliton and optical frequency comb (OFC) can be generated in fiber cavities or microresonators with anomalous group velocity dispersion (GVD), which has achieved great breakthroughs and found significant applications in many fields. Despite recent progress in this field, it remains challenging to obtain OFCs with a flat-top spectrum and a high conversion efficiency in an optical cavity with anomalous GVD. In this paper, intra-cavity dispersion engineering is proposed to reshape the bright dissipative Kerr solitons in a Brillouin laser cavity with anomalous GVD for generating flat-top OFCs with a high conversion efficiency. By introducing a piece of optical fiber with normal GVD into the above Brillouin fiber laser cavity for making a hybrid fiber cavity with near zero net cavity GVD, the intra-cavity waveform is broadened in time-domain due to the existence of normal GVD, which conquers the pulse compression caused by four-wave mixing and anomalous GVD, and leads to the formation of bright dissipative Kerr solitons and OFCs with a flat-top profile and a high conversion efficiency, via interlocking of up-switching waves. Furthermore, flat-top OFCs with step tunable mode spacing from GHz to THz are achieved via intra-cavity dispersion engineering.

Subharmonic instabilities in Kerr microcombs.

We report experimental observation of subharmonic mode excitation in primary Kerr optical frequency combs generated using crystalline whispering-gallery mode resonators. We show that the subcombs can be controlled and span a single or multiple free spectral ranges around the primary comb modes. In the spatial domain, the resulting multiscale combs correspond to an amplitude modulation of intracavity roll patterns. We perform a theoretical analysis based on eigenvalue decomposition that evidences the mechanism leading to the excitation of these combs.

Kerr Frequency Comb and Stimulated Raman Comb Covering S+C+L+U Band Based on a Packaged Silica Spherical Microcavity

Optical microresonators supported whispering gallery modes (WGMs) are one of the most cost-effective platforms for optical frequency comb generated due to their advantages of ultra-high quality ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> ) factor and ultra-low mode volume. In this paper, we report a portable and robust packaged silica spherical microcavity by melting the end of a standard single-mode fiber with ultra-high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> factor up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${10}^8$</tex-math></inline-formula> , and theoretically and experimentally demonstrate a broadband optical frequency comb (OFC) generation including Kerr OFC and stimulated Raman comb. Kerr OFCs are separated by one, two, eight and twelve free spectral ranges corresponding to 1.4, 2.8, 11.2, and 16.8 nm, respectively. The transition between Kerr OFC and stimulated Raman comb is achieved by changing the pump laser power and the detuning frequency resulting from gain competition between modulation instability and Raman gain. Benefitting from the dispersion control and ultra-high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> factor persistence in our packaged microcavity, OFC can be adjusted continuously covering from S-band to C-band, L-band and U-band. We also explain these results by numerical simulations using a model framework combined with the Lugiato-Lefever equation and Raman response function.

Mid-infrared ultra-broadband optical Kerr frequency comb based on a CdTe ring microresonator: a theoretical investigation: erratum.

An erratum to correct a mistake on the authorship in the author list in [Opt. Express30(19), 33969 (2022)10.1364/OE.469599]. The corrections have no influence on the results and conclusions of the original paper.

Kerr comb generation in coaxial MgF2 whispering gallery mode micro-disk resonators

In this work, we proposed coaxial structure MgF2 crystal micro-disk resonators with ultra-high quality factors. Based on such a structure, two wide bands of Kerr optical frequency combs were separately generated in two adjacent micro-disk resonators with a repetition rate of approximately 44.11 GHz. The comb lines reached up to 500 within an approximately 250 nm wavelength band. This compact coaxial structure can be used to generate multi-combs with a low difference in repetition rate and applied to dual-comb spectral measurement applications.

Mid-infrared ultra-broadband optical Kerr frequency comb based on a CdTe ring microresonator: a theoretical investigation.

Microresonator Kerr frequency combs are coherent light sources that emit broadband spectrum of evenly spaced narrow lines in an optical microresonator, which provide breakthroughs in many technological areas, such as spectroscopy, metrology, optical telecommunications, and molecular sensing. The development of mid-infrared (MIR) optical frequency comb (OFC) based on microresonators could pave the way for high performance spectroscopy in the MIR "molecular fingerprint" region. However, the generation of microresonator MIR OFC, especially towards the long-wavelength MIR (>10 µm) region, is prohibited by the transmission window of the commonly used Kerr optical media such as Si and Si3N4, and low nonlinearity at long wavelengths. Here, we seek the possibility to realize an ultra-broadband frequency comb operating in the long-wavelength MIR region based on a cadmium telluride (CdTe) ring microresonator. CdTe features a broad transmission range covering the wavelengths of 1∼25 µm, a flat dispersion profile, and an extraordinary third-order nonlinear refractive index (∼1.4 × 10-17 m2W-1 at 7 µm) which is 2-order greater than that of Si3N4, making it a promising platform to realize MIR Kerr frequency comb. Based on the above excellent optical properties, we design a CdTe/cadmium sulfide (CdS)/Si heterojunction microring resonator to generate an ultra-broadband MIR OFC. Through the numerical simulation, the geometric parameters (width, height, and radius) of the microresonator, polarization, wavelength of the pump, and quality factor are investigated and optimized. As a result, a MIR OFC covering 3.5∼18 µm is numerically demonstrated by using the pump wavelength of 7 µm and a pump power of 500 mW. This is the first simulation demonstration of Kerr OFC with the spectral range extending beyond 10 µm, to the best of our knowledge. This work provides new opportunities for the realization of ultrabroad microresonator frequency combs based on novel Kerr optical medium, which can find important applications ranging from calibration of astronomical spectrographs to high-fidelity molecular spectroscopy.

Platform-Independent Optimization of Pump Power Threshold for Microcomb Generation

Kerr optical frequency combs have found various applications in science and technology, and minimizing their pump power has become an important area of research. These combs are generated using a wide variety of platforms, with a size ranging from micrometers to millimeters, and quality factors ranging from millions to billions. It is therefore not trivial to assess the pump power requirements for comb generation when they have such a large diversity in terms of resonator properties and pump configurations. We propose a suitably normalized threshold pump power as a metric to optimize Kerr comb generation independently of the platform. This method allows one to evaluate the minimum threshold power solely based on the properties of the bare resonator, and independently of dispersion, detuning or coupling considerations. In order to confirm the validity of this approach, we experimentally demonstrate Kerr comb generation in a millimeter-size magnesium fluoride whispering-gallery mode resonator with a threshold pump power of only 1.2 mW, which is one of the lowest pump powers reported to date for a mm-size resonator.

Performance evaluation of silicon-chip-based mid-infrared Kerr optical frequency combs with ridge cross section

We present a complementary metal-oxide-semiconductor (CMOS) compatible platform for on-chip frequency comb generation in the mid-infrared region based on a silicon-on-insulator (SOI) microring resonator with ridge cross section. Flat dispersion tailoring is performed with dispersion variation of 4.04 × 10−6 ps/nm/km by adjusting the geometry parameter and the low-loss SOI microring resonator with total quality factor (Q) up to 106 can be realized at wavelengths from 3.3 to 3.6 µm. Furthermore, the thresholdless frequency combs consisting of 50 comb lines spanning from 3.1 to 4.0 µm (over 900 nm) can be realized using SOI microring resonator with 50 mW pump power. Besides, the study shows that the frequency interval of the comb is related to the selection of the dual-pumped wavelength. The influences of the coupling coefficient and the radius of microring on the bandwidth of mid-infrared OFC are also investigated numerically which shows that remarkable enhancement of mid-infrared OFC bandwidth can reach 449 nm when the coupling coefficient varies from 0.012 to 0.05. This research work is instructive for realizing highly integrated photonics and achieving the experimental generation of mid-infrared optical frequency combs, which could enable substantial progress in spectroscopy applications.

Silica Microsphere WGMR-Based Kerr-OFC Light Source and Its Application for High-Speed IM/DD Short-Reach Optical Interconnects

Kerr optical frequency combs (OFCs) based on silica microsphere whispering gallery mode resonator (WGMR) have various applications where they are used as a light source. For telecommunication purposes, WGMR-based Kerr-OFC comb generators can be physically realized using silica microsphere resonators and can be used to replace multiple laser arrays. In such a realization, these novel light sources have the potential to demonstrate an attractive solution for intra-datacenter interconnects (DCI). In this paper, we show an experimental demonstration of a silica microsphere WGMR-based Kerr OFC light source where newly generated 400 GHz spaced carriers together with powerful linear equalization techniques, such as a linear symbol-spaced adaptive decision-feedback equalizer (DFE) with feed-forward (FF) and feedback (FB) taps, provide an alternative to individual lasers ensuring low-cost and low-complexity IM/DD scheme for the transmission of NRZ-OOK modulated signals at data rates up to 50 Gbps/λ over 2 km SMF link. Finally, we demonstrate a record 50 Gbps per λ transmission of NRZ-OOK modulated signals with a novel silica microsphere WGMR-based Kerr-OFC as a light source operating in the optical C-band, surpassing the previously demonstrated data rate record by five times.

Modulation instability of Kerr optical frequency combs in dual-coupled optical cavities

Kerr optical frequency combs generated in a coherently driven Kerr nonlinear resonator have the potential for a wide range of applications. However, in a single cavity which is a widely adopted configuration for Kerr optical frequency-comb generation, modulation instability is suppressed in the normal dispersion regime, and the pump-to-comb conversion efficiency is extremely low for a single dissipative Kerr soliton (DKS) in the anomalous dispersion regime. Dual-coupled cavities have been proposed to generate Kerr optical frequency combs in the normal dispersion regime, and have the potential to remarkably increase conversion efficiency for Kerr optical frequency combs. Here, we investigate modulation instability and Kerr optical frequency-comb formation in dual-coupled cavities. Based on solutions of the continuous-wave steady state, we obtain a quadric algebraic equation describing the modulation instability gain, and we find that it is intensely influenced by the scaled free spectral range mismatch between the two cavities. Our numerical simulations demonstrate that platicons can be generated via a pump scanning scheme for the case that both the two cavities possess normal dispersion, and a single DKS can be generated in the cavity with anomalous dispersion while the dispersion of the other cavity is normal. The conversion efficiency of a single DKS is determined by the pump power and detunings as well as intrinsic quality factors of two cavities. Our analysis of modulation instability provides a powerful tool for Kerr optical frequency-comb generation via pump modulation and cavity detuning tuning scheme in dual-coupled cavities.

Stability and non-linear dynamic analysis of Kerr optical frequencycombs in dual-coupled microcavities with high-order dispersion

We investigate the stability and nonlinear dynamics of the Kerr optical frequency comb inside a dual coupled microcavity with high-order dispersion effect based on the theoretical model of coupled nonlinear Schrödinger equation. The effects of different high-order dispersion parameters on the evolution and spectral characteristics of the optical field in the cavity are also explored. Theoretical results indicate that the addition of the third-order dispersion enlarges the stability domain of the parametric space and transforms the periodically varying soliton breathers and chaos into stable bright soliton. In order to obtain an accurate Kerr optical frequency comb spectral envelope, higher-order dispersion should be considered. Moreover, high-order dispersion terms have a significant effect on the spectral characteristics of the optical frequency comb, such as the spectral envelope frequency shift and the dispersive wave spectral position. Specifically, the third-order dispersion and positive fourth-order dispersion can broaden the spectrum and enhance the dispersive waves; while the negative fourth-order dispersion can suppress the dispersive wave generation and obtain a symmetric soliton frequency comb; the fifth-order dispersion can regulate the drift direction and speed of the optical solitons. The theoretical results are of great value for dispersion regulation and design and also for stability studies in double-coupled microcavity experiments.

Effect of Thermal Tuning and Mode Coupling On Soliton Microcombs in AlN Microresonators

Optical resonator-based Kerr frequency combs allow an ultra-precise time and frequency measurements for a wide range of applications including ranging, biosensing, high-capacity communications, and quantum information technology. Kerr solitons are coherent frequency combs that are crucial for frequency synthesizer and chronometric instruments. The behavior of the soliton is determined by multiple factors while the key challenge for stable operation is to maintain thermal equilibrium in the resonator. In this work, owing to the stabilizing thermal effect caused by a neighboring auxiliary mode (TE10), we demonstrate the generation of octave-spanning (1100-2380 nm) single-soliton within a wide laser detuning range (up to 16 GHz) by pumping the TE00 mode. The influence of temperature on the microcombs behavior is also explored. Firstly, diverse two-solitons are achieved by adjusting the temperature. Secondly, the soliton access possibility is reduced when further increasing the temperature due to the avoided mode crossing between the pump mode (TE00) and another adjacent fundamental transverse-magnetic mode. At the same time, the spectral bandwidth of the microcomb that originates from TM00 modes is significantly enhanced. Finally, we show that the carrier offset frequency, which is evaluated to be &#x223C;30-40 GHz, increases as the temperature is increased.

Research on generating broad-spectrum Kerr optical frequency comb and dispersion control based on magnesium fluoride crystal micro-resonator

Research on generating broad-spectrum Kerr optical frequency comb and dispersion control based on magnesium fluoride crystal micro-resonator

Chip-scale Kerr optical frequency comb for wavelength-division multiplexing optical fiber communications (Invited)

Chip-scale Kerr optical frequency comb for wavelength-division multiplexing optical fiber communications (Invited)